The present invention relates to a new and improved method of inspecting an airfoil to determine whether or not its dimensions fit within a tolerance range.
Airfoils, such as turbine blades and vanes, have previously been inspected by measuring the airfoil and comparing a cross sectional drawing of the airfoil to a cross sectional drawing of a reference airfoil. In practicing this known method of inspecting an airfoil, a cross section of the airfoil is measured to determine the location of selected points on the major side surfaces of the airfoil. These measurements are used to make a drawing representing the measured cross sectional configuration of the airfoil on an enlarged scale. In addition, a transparent overlay drawing is made, on the same scale, representing the desired airfoil configuration.
The transparent overlay of the reference airfoil is then manually moved relative to the drawing of the measured airfoil until the lines representing the major side surfaces of the measured airfoil have, in the opinion of the inspector, a best possible fit with the lines representing the major side surfaces of the reference airfoil. The distances by which preselected points on the overlay are offset from corresponding points on the reference drawing are then measured These measurements are checked to determine whether or not the cross sectional configuration of the measured airfoil corresponds to the cross sectional configuration of the reference airfoil within a preselected tolerance range.
This manual method of comparing cross sectional drawings of measured and reference airfoils is a slow and time consuming task. The manual checking process has an accuracy of, perhaps, 1/64th of an inch. Moreover, this manual inspecting process can not be repeated with a high degree of accuracy due to the subjectivity of the person comparing the two cross sectional drawings to determine what is a best fit between the overlay of the measured airfoil and the drawing of the reference airfoil. The degree of repeatability of the manual inspecting process is also adversely affected by judgments which must be made in measuring the distances between points on the overlay and reference drawings. Of course, the lack of repeatability makes it difficult for different parties to check whether or not an airfoil has been inspected properly and is actually within desired tolerances.